Reversal development of latent electrostatic images on xeroprinting masters

ABSTRACT

Process for reversal development of a latent electrostatic image in a layer on a conductive support by developing with an electrostatic developer having electrostatically charged toner particles by 
     (a) generating imagewise areas in the layer having different rates of charge decay and/or charge acceptance, 
     (b) charging the layer, 
     (c) allowing formation of an electrostatic image corresponding to the imagewise generated areas by differential charge decay and/or charge acceptance, 
     (d) creating an electrical field to attract toner particles preferentially to the areas of lesser charge, and 
     (e) developing the areas of lesser charge with electrostatically charged toner particles having the same polarity as that of the charged layer. 
     The developed image can be transferred to a receptor surface, e.g., paper. The process is useful with many type photosensitive masters in preparing reversal images with the use of only one master, toner and film original.

DESCRIPTION

1. Technical Field

This invention relates to a process for the reversal development oflatent electrostatic images. More particularly, this invention relatesto a reversal development process wherein the latent image is formed ina photohardenable, leuco dye containing photosensitive, wash-outphotohardenable, or silver halide salt-based electrostatic element.

2. Background of the Invention

Xeroprinting masters are elements with an image or pattern of lowconductivity material on a conduct support. Typically the image has beenproduced by exposure of the element to actinic radiation through a filmoriginal, in some cases followed by chemical processing. Charging theelement, for example by corona discharge, produces an electrostaticimage corresponding to the image or pattern of the low conductivitymaterial. The electrostatic image is developed by toning with oppositelycharged toner particles, and the toned image can be transferredelectrostatically or by other means to a receptor such as paper or film.Either dry or liquid developers can be used.

The image or pattern of low conductivity material in the xeroprintingmaster is permanent or persistent, so after transfer of toner to areceptor, such as paper, the master can be returned for a secondprinting cycle. Multiple charge, tone, transfer cycles, and thusmultiple copies on receptors, can be made from a single exposure orimaging step. This multiple printing capability distinguishesxeroprinting masters from photoconductors commonly used inelectrophotography,

The functionality of xeroprinting masters in electrography depends onrates of decay and/or acceptance of electrostatic charge in certainregions being different from rates in other regions. The differentrelative rates are manifestations of chemical differences between thetwo regions (imaged and non-imaged) inherent in xeroprinting masters.These differences distinguish xeroprinting masters from photoconductorswherein imaged regions are not chemically different from non-imagedregions. Both differences in rates of charge decay and differences inrates of charge acceptance between imaged and non-imaged regionsmanifest themselves in differences in electrostatic charge in thoseregions.

A number of xeroprinting masters have been developed. In some, regionsof low conductivity correspond to imaged areas; in others, regions oflow conductivity correspond to non-imaged areas. With certainphotopolymerizable (photohardenable) elements, for example, exposurecreates areas of reduced conductivity. These elements will be referredto as "negative-working." With certain silver halide salt-basedelements, on the other hand, exposure and processing creates areas ofenhanced conductivity. These elements will be referred to as"positive-working." When xeroprinting masters are used conventionally,as described above in the first paragraph of this section, the regionsof low conductivity, i.e., regions with the slower rate of charge decayor higher rate of charge acceptance, retain electrostatic charge andhence attract oppositely charged electrostatic toner. The resultingtoned images are thus images of the regions of low conductivity. Toproduce desirable toned and printed positives, however, a xeroprintingmaster is limited to use with a single type of film original (positivevs. negative). Using a photopolymerizable element as a xeroprintingelement, for example, a negative film original is required; alternatelyusing a silver halide salt-based element, e.g., as described above, apositive film original is required. To obtain a printed positive from anegative film and a positive-working master, or from a positive film anda negative-working master, would require an intermediate photoreversalstep and the necessity of producing a second film for use in exposure ofthe xeroprinting element.

It is desirable to directly produce toned images which arephotoreversals of those described above without the need for creating asecond film original. This would allow production of printed positivesdirectly from positive film originals as well as from negative filmoriginals. For xeroprinting masters, such a reversal requires toning ofthe areas of the element having lesser charge rather than the areashaving greater charge as in conventional use. Such a reversal wouldenable the generation of a positive-to-positive imaging system utilizingthe same masters and toners to be used for a negative-to-positiveconventional process, or would enable a negative-to-positive imagingsystem utilizing the same masters and toners to be used for apositive-to-positive conventional system.

The process of the invention allows for the generation of both positiveand negative images using a single electrostatic master and toner andeither a positive or negative film original.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided a process forreversal development of a latent electrostatic image in a layer on aconductive support by developing with an electrostatic developer havingelectrostatically charged toner particles by

(a) generating imagewise areas in the layer having different rates ofcharge decay and/or charge acceptance,

(b) charging the layer,

(c) allowing formation of an electrostatic image corresponding to theimagewise generated areas by differential charge decay and/or chargeacceptance,

(d) creating an electrical field to attract toner particlespreferentially to the areas of lesser charge, and

(e) developing the areas of lesser charge with electrostatically chargedtoner particles having the same polarity as that of the charged layer.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention with reversal development, the samexeroprintingelectrostatic elements or masters and the same toners can beused to produce positive toned images and thus positive prints fromeither positive or negative film originals. Without reversal developmenttwo different masters would be required to be able to print positivesfrom either film original, or secondary films would have to be created.

In conventional use areas of low conductivity, areas which retainelectrostatic charge, are developed. Toned density uniformity isdependenton electrostatic charge uniformity, which in turn is dependenton material and system uniformity. With reversal development, however,areas with little or no electrostatic charge, are developed. Uniformityin toned density is not dependent on uniformity of charge in the lowconductivity regions.

In the process for reversal development of the invention the latentelectrostatic image may be present in a layer that is photohardenable,leuco dye-containing photosensitive, wash-out photohardenable, or silverhalide-based electrostatic element. Other elements not exemplified heremay be used provided they are capable of generating imagewise areashavingdifferent rates of charge decay and/or charge acceptance.

The photohardenable electrostatic element or master comprises aphotohardenable layer on a conductive support. A cover sheet, e.g.,plastic film, may be present on the photohardenable layer. Thephotohardenable (photopolymerizable) layer of the electrostatic elementconsists essentially of at least one organic polymeric binder, at leastone compound having at least one ethylenically unsaturated group whichcanbe a monomer, a photoinitiator or photoinitiator system, optionally achaintransfer agent as well as other additives, and optionally either(1) at least one organic electron donor, also known as p-type conductingcompoundor at least one organic electron acceptor, also known as ann-type conducting compound as described in Blanchet-Fincher et al. U.S.Pat, No. 4,849,314, or (2) a substituted aromatic amino compound, andpreferably a strong acid as described in Blanchet-Fincher, Fincher,Cheung, Dessauer and Looney, U.S. Pat. No. 4,818,660. Preferably thechain transfer agent is present. Photohardenable electrostatic elementswith improved environmental latitude are disclosed in Blanchet-Fincherand Chang, U.S. Ser. No. 351,361, filed May 12, 1989.

Throughout the specification the below-listed term has the followingmeaning:

"Consisting essentially of" as used in this specification and claimsmeans that there can be present in the photohardenable layer, inaddition to theprimary ingredients, other ingredients which do notprevent the advantages of the invention from being achieved. These otheringredients which can also be present are set out below. Polymericbinders, ethylenically unsaturated compounds, photoinitiators, includingpreferred hexaarylbiimidazole compounds (HABI's) and chain transferagents are disclosed in Chambers U.S. Pat. No. 3,479,185, Baum et al.U.S. Pat. No. 3,652,275, Cescon U.S. Pat. No. 3,784,557, Dueber U.S.Pat. No. 4,162,162,and Dessauer U.S. Pat. No. 4,252,887, the disclosuresof each of which, as well as the two U.S. patents and one U.S. patentapplication set out above, are incorporated herein by reference.

Positive working electrostatic elements having a photosensitive layer,specifically a leuco dye containing photosensitive layer, on aconductive support are disclosed in Kempf, Dessauer and Froelich, U.S.Ser. No. 07/374491, filed June 30, 1989 now U.S. Pat. No. 4,945,020, thedisclosureof which is incorporated herein by reference.

Photohardenable wash-out layers, for example, that may be coated on orlaminated to a conductive support to form an electrostatic master aredisclosed in Chen U.S. Pat. No. 4,323,636, Chen and Brennan U.S. Pat.No. 4,323,637, Fan U.S. Pat. No. 4,072,527, Bratt U.S. Pat. No.4,072,528, andAlles U.S. Pat. No. 3,458,311, the disclosures of whichare incorporated herein by reference.

The silver halide salt-based electrostatic element are disclosed inCairncross, U.S. Pat. No. 4,868,081 the subject matter of which isincorporated herein by reference. This patent discloses a photosensitivecomposition consisting essentially of a silver halide photographic saltdispersed in a synthetic insulating polymeric binder that is swellablein aqueous solutions having a pH greater than about 81/2, saidcomposition having an insulating value such that it will support amacroscopic electric field of at least approximately 5 volts/μm asmeasured 2 seconds following full charging of its surface that has beenallowed to equilibrate at 50% relative humidity for 1 hour.

The photohardenable (photopolymerizable), and wash-out photohardenableelements are exposed imagewise by actinic radiation whereby the exposedareas become hardened or polymerized generating imagewise areas havingdifferent rates of charge decay and/or charge acceptance. Suitableradiation depends on the sensitivity of the particularphotopolymerizable layer composition used to form the photopolymerizablelayer. Generally standard ultraviolet energy sources are used. If,however, the photopolymerizable composition is sensitive to visiblelight then that type of exposure source can be used. Exposure sourcescan also be of the laser type. The exposing radiation can be modulatedeither by digital or analog means. Analog exposure utilizes a line orhalf-tone negative or other pattern interposed between the radiationsource and photopolymerizable layer. Digital exposure is by means of acomputer controlled visible light-emitting laser which can scan the filmin raster fashion. For digital exposure a high speed photopolymerizableelement is utilized, e.g., one containing a high-level ofhexaarylbiimidazole photoinitiator, chain transfer agent and sensitizedto higher wavelength light with a sensitizing dye.

The silver halide salt-based electrostatic element is exposed imagewiseusing any of the procedures commonly used with silver halidephotographic materials, such as by imaging with actinic light, cathoderay tube, or laser. In the case of films consisting of silver halidegrains dispersed in an insulating binder, the latent image is thendeveloped by reducing the exposed silver halide particles to metallicsilver using conventional aqueous developing solutions. A conventionalaqueous fixing solution, suchas sodium thiosulfate, is then used toremove the unexposed silver halide particles. The developed elementhaving imagewise areas having different rates of charge decay and/orcharge acceptance is then ready for the electrostatic printing process.In the case of diffusion transfer silver halide films, the latent imageis developed to give silver metal in the silver halide emulsion layerand unexposed silver halide is dissolved withcomplexing agents. Thecomplexed unexposed silver halide then diffuses intothe underlyinginsulating polymer layer which contains development nuclei, wherein thesilver ions are reduced to silver metal on the development nuclei. Theemulsion layer is then removed by washoff processing to give anelectrostatic master ready for printing.

The leuco dye containing photosensitive layer is exposed to radiation ofwavelength in the 200 to 500 nm range preferably about 310 to about 400nm, and most preferably about 360 nm. Any convenient source ofultraviolet/visible light may be used to activate the light-sensitivecomposition and induce the formation of an image. In general, lightsources that supply radiation in the region between about 2000 Å andabout 5000 Å are useful in producing images. Among the light sourceswhich can be employed are sun lamps, electronic flash guns, germicidallamps, carbon arcs, mercury-vapor arcs, fluorescent lamps withultravioletemitting phosphors, argon and xenon glow lamps, electronicflash units, photographic flood lamps, ultraviolet lamps providingspecifically light of short wavelength (2537 Å) and lamps providinglight of long wavelength (4500 Å). The light exposure time will varyfrom a fractionof a second to several minutes depending upon theintensity of the light, its distance from the photosensitivecomposition, the opacity of the phototool, and the nature and amount ofthe photosensitive composition. There may also be used coherent lightbeams, for example, pulsed nitrogen lasers, argon ion lasers and ionizedNeon II lasers, whose emissions fall within or overlap the ultravioletabsorption bands of the HABI. Visible light emitting lasers such asargon ion, krypton ion, helium-neon, and frequency doubled YAG lasersmay be used for visibly sensitized photosensitive layers.

Ultraviolet emitting cathode ray tubes widely useful in printout systemsfor writing on photosensitive materials are also useful for imaging thesubject compositions. These in general involve a UV-emitting phosphorinternal coating as the means for converting electrical energy to lightenergy and a fiber optic face plate as the means for directing theradiation to the photosensitive target. For purposes of this invention,the phosphors should emit strongly below 420 nm (4200 Å) so as tosubstantially overlap the near UV-absorption characteristic of thephotosensitive compositions of the invention. Representative phosphorsinclude the P4B (emitting at 300-550 nm, peaking at 410 nm), P16(330-460 nm, peaking at 380 nm) and P22B (390-510 nm, peaking at 450 nm)types. Electronic Industries Association, New York, NY assigns P-numbersand provides characterizing information on the phosphors; phosphors withsame P-number have substantially identical characteristics.

Prior to or after the imagewise exposure the cover sheet, if present,can be removed by stripping or peeling as is known to those of ordinaryskill in the art.

After the imagewise areas having different rates of charge decay and/orcharge acceptance have been generated in the photohardenable, wash-outphotohardenable, leuco dye-containing photosensitive or silver halidesalt-based electrostatic element, the layer containing the imagewisegenerated areas is electrostatically charged, and then allowed to forman electrostatic image corresponding to the imagewise generated areas.The elements may be allowed to stand for 0.001 to 10.0 minutes,preferably 0.01 to 0.25 minute to differentially discharge, depending onthe nature of the xeroprinting element. The preferred electrostaticcharging means iscorona discharge via a scorotron. Alternatively,charging can be accomplished with the use of a shielded corotron,radioactive source, contact electrodes such as electrically biasedsemiconductive rubber rollers, and the like.

An electrical field is then created to attract toner particlespreferentially to the areas of lesser charge. When the imagewise areasin the layer having different rates of charge acceptance are generatedand the layer is charged, the electrical field may be createdimmediately thereafter. For controlled development of electrostaticimages a development electrode, typically a conductive plate or aconductive roll parallel and close to the xeroprinting element, isemployed.

In conventional charged area development the development electrode ismaintained at a potential which is of the same sign but small relativeto the potential of the charged areas of the xeroprinting element. Atoner isemployed which has a charge of opposite sign from the charge ofthe element. Toner present in the field between the element and thedevelopment electrode is thus attracted to the areas of greater chargeof the element.

In reversal development, however, the xeroprinting element is chargedwith the same polarity as the charge of the toner to be used. Anelectric fieldis created between the development electrode and thexeroprinting element by applying a voltage bias to either thedevelopment electrode or the conductive backing of the xeroprintingelement. The voltage is adjusted toproduce a potential on the electrodeless than the potential on the elementin charge retaining areas butgreater than the potential in the discharged areas. In insulating,charge-retaining areas no development of the xeroprinting elementoccurs. However, the field created between the electrode and the elementin areas of the element of lesser charge attracts toner to thexeroprinting element in these areas.

The areas of lesser charge are then developed by means of anelectrostatic dry toner or liquid electrostatic developer, the latterbeing preferred. Dry electrostatic toners are known to those skilled inthe art. Known electrostatic liquid developers and known methods ofdeveloper applicationcan be used. Preferred liquid electrostaticdevelopers are suspensions of pigmented resin toner particles innonpolar liquids which are generally charged with charge directorcompounds, e.g., ionic or zwitterionic compounds. The nonpolar liquidsnormally used are the Isopar® branched-chain aliphatic hydrocarbons(sold by Exxon Corporation) which have a Kauri-butanol value of lessthan 30 and optionally containing various adjuvants as described inMitchell U.S. Pat. Nos. 4,631,244 and 4,663,264, Taggi U.S. Pat. No.4,670,370, Larson and Trout U.S. Pat. No. 4,681,831, El-Sayed and TaggiU.S. Pat. No. 4,702,984, Larson U.S. Pat. No. 4,702,985, Trout U.S. Pat.No. 4,707,429, and Mitchell U.S. Pat. No. 4,734,352. The disclosures ofthese patents are incorporated herein by reference. The above nonpolarliquids are narrow high-purity cuts of isoparaffinic hydrocarbonfractions with the following boiling ranges: Isopar®-G 157°-176° c.;Isopar®-H 176°-191° C.; Isopar®-K 177°-197° C.; Isopar®-L 188°-206° C.;Isopar®-M 207°-254° C.; Isopar®-V 254°-329° C. Otherknown hydrocarbonliquids can be used as well. Preferred resins of the liquidelectrostatic developers are copolymers of ethylene (80 to99.9%)/acrylic or methacrylic acid (0 to 20.0%)/alkyl of acrylic ormethacrylic acid where alkyl is 1 to 5 carbon atoms (0 to 20%), e.g.,copolymers of ethylene (89%) and methacrylic acid (11%) having a meltindex at 190° C of 100. Other resins disclosed in the above UnitedStates patents are also useful. The disclosure relating to resins fromthese patents is incorporated herein by reference. The resin tonerparticles preferably have an average particle size of (by area) lessthan 10 μm, as measured by a Horiba CAPA-500 centrifugal particleanalyzer, Horiba Instruments, Inc., Irvine, Calif. Preferred nonpolarliquid solubleionic or zwitterionic components which in general affordnegatively chargedtoner, are lecithin and Basic Barium Petronate®oil-soluble petroleum sulfonate manufactured by Sonneborn Division ofWitco Chemical Corp., New York, N.Y., Emphos® anionic glycerides, sodiumsalts of mono- and diglycerides with saturated and unsaturated acidsubstituents, also manufactured by Witco Chemical Corp., NY, N.Y. Manyof the monomers usefulin the photohardenable composition are soluble inthese Isopar® hydrocarbons, especially in Isopar®-L, as well as othernonpolar liquids. Consequently, repeated toning with Isopar® baseddevelopers to make multiple copies can deteriorate the electricalproperties of the master by extraction of monomer from unexposed areas.The preferred monomers are relatively insoluble in Isopar® hydrocarbons,and extended contact with these liquids does not unduly deterioratefilms madewith these monomers. Photopolymerizable electrostatic elementsmade with other, more soluble monomers can still be used to makemultiple copies, using liquid developers having a dispersant with lesssolvent action.

After toning with dry toner developers or developing with liquidelectrostatic developer the developed image can be transferred toanother surface or receptive support, such as paper, for the preparationof an image. Other receptor supports include, but are not limited, topolymeric films, cloth or other printable materials and surfaces. Formaking integrated circuit boards, the transfer surface can be aninsulating boardon which conductive circuit lines can be printed by thisprocess, or it canbe an insulating board covered with a conductor, e.g.,a fiber glass board covered with a copper layer, on which a resist isprinted by this process.Transfer is accomplished by electrostatic orother means, e.g., by contact with an adhesive receptor surface orapplying pressure and heat, or a combination of these methods.Electrostatic transfer can be accomplished in any known manner, e.g., byplacing the receptive support on a conductive cylinder and bringing thetoned surface within 0.002 to 0.1 inch (0.05 to 2.54 mm) of the paper,the gap being filled with Isopar®hydrocarbon. When negatively chargedtoner particles are used, a positive potential is applied to theconductive cylinder, driving the toner particles of the developer offthe photohardenable electrostatic master onto the receptive support,e.g., paper. Alternately, the paper may be placed in contact with thedeveloped image using a tackdown roll or coronawhich when held atnegative voltages ill press the two surfaces together assuring intimatecontact. After tackdown a positive corona discharge is applied to thebackside of the paper driving the toner particles of the developer offthe photohardenable electrostatic master onto the paper. In the case ofpositively charged toners, polarities opposite to that described aboveare used to effect tone transfer. In making multiple images from asingle imagewise exposed photohardenable electrostatic master, it isonly necessary to repeat the steps of charging electrostatically, toningand transferring. Each transfer requires a separate receptor support orsurface.

INDUSTRIAL APPLICABILITY

The reversal process is particularly useful in the graphic artsindustry, particularly in the area of color proofing wherein the proofsprepared duplicate the images achieved by printing. The process of theinvention satisfies the proofing needs of all printers whether they workwith positive or negative color separations because the process allowsone master and one separation to produce both positive or negativeimages. Theprocess is also useful in making integrated circuit boardsand printing plates.

EXAMPLES

The following examples illustrate but do not limit the invention whereinthe percentages are by weight.

EXAMPLE 1

An electrostatic printing master was prepared by dispersing aconventional silver halide emulsion in an insulating polymer and coatingthe mixture ona conductive substrate in a manner similar to thatdescribed in Example 12 of Cairncross U.S. Pat. No. 4,868,081 with theexception that the insulating binder contained "Polymer E" (Example 5 ofU.S. Pat. No. 4,868,081) and indium tin oxide coated polyester was usedas the substrate. The film was contact exposed through a high resolutionpositivephototool and tray processed (develop, fix, stop, rinse and dry)as described in Example 12 of U.S. Pat. No. 4,868,081. The resultantimage consisted of conductive silver areas where the film was exposed(background areas) and insulating silver-free areas where the film wasunexposed.

The film was mounted on a flat aluminum plate and an electricalconnection between the conductive indium tin oxide (ITO) substrate ofthe master and the aluminum plate was made with the use of conductivecopper foil tape (Chomerics, Inc., Hudson, NH). The aluminum plate wasthen electrically connected to a DC power supply. With the mastergrounded through the powersupply, the film was corona charged with a 12inch (30.48 cm) long, single wire corotron operated at +6 kV. A secondparallel aluminum plate was mounted above the charged master to serve asa development electrode. The two aluminum plates were separated by aspacing of 0.075 inch (0.1905 cm) with insulating posts. The plate withthe master (and hence the ITO substrate) was biased with -50 V. Thedevelopment electrode was biased with -20 V (optional). The assembly wasthen placed in a plastic tray containing a positively charged liquidelectrostatic toner (James River Graphics T1818) for a period of 3seconds, after which the plates were removed from the bath and excesstoner allowed to drain. The plates were separated and the biases turnedoff. Toner was found to have preferentially deposited on the conductiveportions (exposed areas) of themaster. Toner was then electrostaticallytransferred from the master to paper with the use of a bias rolloperated at -1 kV. The toner image was then dried and fused on the paperat approximately 100° C. in an oven.

EXAMPLE 2

An electrostatic master was prepared from a silver halide diffusiontransfer film coated on a conductive substrate (ITO described inExample 1) in a manner similar to that described in Example 29 ofCairncross, U.S.patent application 07/196,803 filed May, 16, 1988, nowU.S. Pat. No. 4,868,081 with the exception that the insulating bindercontained "PolymerE" (Example 5 of U.S. patent application 07/196,803).The film was contact exposed through a negative phototool and trayprocessed (develop, stop, wash-off silver halide emulsion layer, rinseand dry) in a manner similar to that described in Example 29 of U.S.application 07/196,803, now U.S. Pat. No. 4,868,081 with the exceptionthat the developer contained an additional 12.5% by weight potassiumhydroxide. The resultant image consisted of conductive silver areaswhere the film was unexposed and electrically insulating silver-freeareas where the film was exposed.

The film was mounted on an aluminum drum and the drum, in turn, mountedin a modified Savin 870 copier. An electrical connection was madebetween theconductive substrate (ITO) of the master and a DC powersupply. With the master grounded through the power supply, the film wascorona charged withthe corotron operated at -6 kV. After charging, thesubstrate of the masterwas biased with +20 V. The development electrodewas maintained at ground potential. Rotation of the drum bearing themaster (2 rpm) through the development station containing negativelycharged liquid electrostatic toner pigmented with carbon black similarto that described in Control 1 of Mitchell, U.S. Pat. No. 4,631,244,resulted in a developed image with the developer preferentiallydeposited on the conductive silver areas (unexposed areas) of themaster. The developer was transferred to paper via a bias roll operatedat +750 V. The developed image was then dried andfused on the paper atapproximately 100° C. in an oven.

EXAMPLE 3

A CitiPlate® master (washoff photopolymer on a flexible aluminumsubstrate) was mounted in a modified Savin 870 copier as described inExample 2. With the aluminum substrate grounded, the master was coronacharged (corotron operated at -6 kV). A bias of +25 V was then appliedto the substrate. The development electrode was maintained at groundpotential. Rotation of the drum bearing the master (2 rpm) through thetoning station containing negatively charged black liquid electrostatictoner similar to that described in Example 2 gave a developed image withdeveloper preferentially deposited on the bare aluminum areas (unexposedareas where photopolymer was washed off) of the master. The developedimage was transferred to paper with the use of a bias roll operated at+550 V. The image was then dried and fused on the paper at approximately100° C. in an oven.

EXAMPLE 4

A photopolymerizable composition consisting of 57.0%poly(styrene/methylmethacrylate)(70/30), 28.6% ethoxylatedtrimethylolpropane triacrylate, 10.6%2.2',4,4'-tetrakis(o-chlorophenyl)-5,5'-bis(m,p-dimethoxyphenyl)-biimidazole,and 3.8% 2-mercaptobenzoxazole was coated on a 0.004 inch (0.0102 cm)aluminized polyethylene terephthalate film substrate. A 0.00075 inch(0.0019 cm) polypropylene cover sheet was laminated to the driedphotopolymerizable layer. The photopolymerizable element was exposedimagewise (for 4 integrated intensity units) through a halftone positivefilm with its emulsion side in contact with the cover sheet, using aDouthitt Option X exposure type 5027 lamp, Douthitt Corporation,Detroit, Mich. The aluminized substrate with the imagedphotopolymerizable element was mounted on a flat plate and the coversheet was then removed. The aluminized substrate was electricallygrounded. The photopolymerizable element was then charged negatively bypassing at 0.5 inch/sec (1.27 cm/sec) over a corotron operated at 4.25kV. A positive potential of +200 V was then applied to the aluminizedsubstrate, and the element was toned (approximately 30 seconds aftercharging) with negatively-charged black liquid electrostatic developer.A 0.04 inch (0.1016 cm) developer-filled gap between a flat developmentplate, held at electrical ground, and the charged photopolymerizableelement, was used.

The black developer was prepared using the following procedure: In aUnion Process 1-S Attritor, Union Process Company, Akron, Ohio, wereplaced the following ingredients:

    ______________________________________                                        Ingredient            Amount (g)                                              ______________________________________                                        Copolymer of ethylene (89%) and                                                                     200                                                     methacrylic acid (11%), melt index at                                         190° C. is 100, acid no. is 66                                         Sterling ® NS carbon black                                                                      25.6                                                    Cabot Corp., Boston, MA                                                       Heucophthal Blue G XBT-583D                                                                          1.6                                                    Heubach, Inc., Newark, NJ                                                     L, nonpolar liquid having                                                                           1000                                                    a Kauri-butanol value of 27,                                                  Exxon Corp                                                                    ______________________________________                                    

The ingredients were heated to 100° C.-110° C. and milled at a rotorspeed of 230 rpm with 0.1875 inch (4.76 mm) diameter steel balls for twohours. The attritor was cooled to room temperature while the milling wascontinued and then 700 grams of Isopar®-H, nonpolar liquidhaving aKauri-butanol value of 27, Exxon Corporation, were added. Milling wascontinued at a rotor speed of 330 rpm for 19 hours to obtain tonerparticles with an average size of 1.5 μm by area. The particulatemediawere removed and the dispersion of toner particles then diluted to2.0 percent solids with additional Isopar®-H. To 2000 grams of thisdispersion were added 12 grams of a 10% solution of lecithin (FischerScientific, Pittsburgh, Pa.) in Isopar®-H.

A black toned image on the photopolymerizable element resulted. Thetoned image was optically positive reproduction of the original halftonepositive film used in imaging the photopolymerizable element. The tonedimage had clean background areas, high image density (1.2-1.4 densityunits after drying), and halftone dots of 3-85% (150 line/inch screen).

EXAMPLE 5

The photopolymerizable element described in Example 4 was exposedimagewise(for 16 integrated intensity units) as described in Example 4through a halftone positive film. The polyethylene terephthalate filmsubstrate was mounted on a flat plate, and the cover sheet was thenremoved from the photopolymerizable element.

The imaged photopolymerized element was charged negatively by passing at0.5 inch/second (1.27 cm/second) over a -4.0 kV corotron, the aluminizedsubstrate being electrically grounded. The element was then toned(approximately 16 seconds after charging) with negatively-charged blackliquid electrostatic toner described in Example 4, by passing over aflat development plate, using a 0.04 inch (0.10 cm) toner-filled gapbetween the charged photopolymerizable element and the developmentplate. In this example a negative potential (-25V) was applied to thedevelopment plate, while the aluminized substrate of thephotopolymerizable element was held at electrical ground.

A black toned image on the photopolymerizable element resulted. Thetoned image was an optically positive reproduction of the originalpositive halftone films used to image the element. The toned imageexhibited clean background areas and halftone dots of 2-85% (150line/inch screen).

The toned image was electrostatically transferred to paper using a biasroll. Plainwell Solitaire offset enamel paper (Plainwell Co., Plainwell,Mich.) was wrapped around a metal drum to which +500 V was applied. Thetoned photopolymerizable element was spaced 0.006 inch (0.015 cm) fromthepaper, the gap being filled with Isopar®-H. Transfer was carried outat0.5 inch/second (1.27 cm/second). The paper was removed from the biasroll and was heated at 110° C. for 1 minute to fuse the toned image andfix it to paper. The image exhibited good solid area density of 1.2-1.4density units.

EXAMPLE 6

A 4 inch (10.16 cm) by 5 inch (12.7 cm) sample of photosensitive filmconsisting of a metallized polyethylene terephthalate support,photosensitive layer, and a polypropylene cover sheet, as described inKempt, Dessauer and Froelich, U.S. patent application Ser. No.07/374,591,filed June 30, 1989 entitled "Photosensitive LeucodyeContaining Electrostatic Master With Printout Image", now U.S. Pat. No.4,945,020 wasimagewise exposed for 20 seconds through a positivehalftone film in emulsion to cover sheet contact in a vacuum frameexposure unit (Douthitt Model X with Theimer Violux® lamp withphotopolymer bulb and Kokomo glass 360 nm ultraviolet light bandpassfilter, Douthitt Corp., Detroit, Mich.). The back surface of the filmwas affixed to a correspondingly sized flat plate of aluminum metal andthe cover sheet removed. A small (0.25 inch (0.635 cm) ×1 inch (2.54cm)) region of the photosensitive layer was removed with a cotton swabsaturated with acetoneto reveal the aluminum surface of the supportsubstrate. This back surface contact to the photosensitive layer waselectrically connected to the aluminum metal plate using a copper metaltape. The plate was in turn connected by a wire to earth ground andpositioned into a parallel rail assembly which supported the side edgesof the plate and held the planar surface at a fixed distance from thegrid of an opposing multi-wire scorotron charging device. The grid ofthis was driven to a potential of -85 Volts and the wire portion to-5.08 kiloVolts. The plate bearing the film sample facing the chargingdevice was moved by hand along the rails to negatively charge thesurface of the film.

The plate was then quickly moved further along the rail assembly toposition the film sample directly opposite a development electrodeconsisting of flat aluminum plate parallel to the film plane and spaced0.007 inch (0.0178 cm) from the surface of the film. This electrode wasdriven to a potential of -110 Volts and the gap between the two surfaceswas filled with negatively-charged black electrostatic liquid developer.After the liquid developer was drained from between the surfaces the gapwas re-filled with clear Isopar®-L nonpolar liquid and the potential ofthe electrode driven to +10 Volts for 10 seconds after which theparallel surfaces were separated by lifting the film supportive plateawayfrom the development electrode and supportive rail assembly.

The resultant toned optically negative image exhibited high uniformity,an absence of background toning, high edge definition, and halftone dotreproduction ranging from 4% to 96% area coverage dots at 150 lines/inchscreen ruling.

The black electrostatic liquid developer was prepared using thefollowing procedure: In a Union Process 30-S Attritor, Union ProcessCompany, Akron,Ohio were placed the following ingredients:

    ______________________________________                                        Ingredient            Amount (Kg)                                             ______________________________________                                        Copolymer of ethylene (89%) and                                                                     5.94                                                    methacrylic acid (11%), melt index                                            at 190° C. is 100, acid no. is 66                                      Sterling ® NS carbon black                                                                      0.7695                                                  Cabot Corp., Boston, MA                                                       Heucophthal Blue G XBT-583D                                                                         0.0405                                                  Heubach, Inc., Newark, NJ                                                     L, nonpolar liquid having                                                                           45                                                      a Kauri-butanol value of 27, Exxon                                            Corporation                                                                   ______________________________________                                    

The ingredients were heated in the range of 90° to 115° C. and milledwith 0.1875 inch (4.76 mm) diameter carbon steel balls for one hour. Theattritor was cooled to room temperature while the milling was continued.Milling was continued for an additional 19 hours. The particulate mediawere removed and Gasic Barium Petronate®, (Witco Chemical Corp.,Sonneborne Division, New York, N.Y.) was added at a level of 30 mg pergram of developer solids. The developer was diluted to 1.5% solids byweight with Isopar®-L for use as an electrostatic liquid developer.

A 12 inch (30.48 cm)×16 inch (40.64 cm) sample of a photopolymerizablefilm consisting of metallized polyethylene terephthalate support,photosensitive layer, and a cover sheet, similar tothat described inBlanchet-Fincher and Fincher, U.S. Pat. No. 4,849,314 wasimagewiseexposed for 10 seconds through optically positive halftone and line artfilms as described in Example 6 above. The sample was then affixed to amechanized drum fixture wherein the drum surface may be rotated througha sequence of positions bearing functional components for electrostaticprocessing of films mounted on said surface. The drum was electricallyconnected to earth ground and the metallic back contact layerof the filmsample was connected to the drum. The drum was rotated at 2 inch (5.08cm) per second surface speed. The film sample travelled past a chargingscorotron device with a constant grid potential -120 Volts and aconstant wire current of 300 microAmperes with wire potential variablein the range of -4.5 to -5.5 kiloVolts. The negatively charged sampletravelled 4 inches (10.16 cm) and entered a two-roll development housingwhere negatively charged magenta pigmented electrostatic liquiddeveloper,similar to that described in Trout, U.S. Pat. No. 4,707,429,Example 5, wasdelivered by pump-fed manifolds to fill the 0.006 inch(0.152 mm) gap between the sample surface and the two developmentelectrode rollers whichwere driven to a potential of -210 Volts androtating to match the surface speed of the sample. Upon exiting thedevelopment housing the toned samplepassed through a counter-rotatingroller electrode held at +25 Volts and spaced 0.003 inch (0.076 mm) fromthe sample surface.

The developed sample then was brought into contact with a sheet of 60#basis weight Solitaire® offset enamel paper (Plainwell Co.,Plainwell,Mich.) by the application of a conductive rubber coated rollerto the back surface of the paper under the influence of gravity anddriven to a constant potential of -3.0 kiloVolts. The paper and samplethen passed under a corotron charging device with a constant wirecurrent of 50 microAmperes and a wire potential of approximately +5kiloVolts. As it exited the corotron device region the paper wasstripped away from the photopolymerizable film sample by hand resultingin complete transfer of the toner layer from the sample surface to thepaper. The paper sheet was dried in an air oven at 105° C. for one totwo minutes to fix the toner image layer to the paper surface. Theresultant optically positive magenta image on paper was of high quality,with good uniformity of solid area coverage at 1.34D, low printbackground density of 0.5D, no density enhancement of feature edges, andhalftone dot reproduction of 1% to 97% dot areas based on 150 lines perinch screen ruling.

We claim:
 1. A process for reversal development of a latentelectrostatic image in a layer on a conductive support by developingwith an electrostatic developer having electrostatically charged tonerparticles comprising in order(a) exposing imagewise to generatepermanent persistent areas in the layer having different rates of chargedecay and/or charge acceptance, (b) charging the layer, (c) allowingformation of an electrostatic image corresponding to the exposedimagewise generated areas by differential charge decay and/or chargeacceptance, (d) creating an electrical field to attract toner particlespreferentially to the areas of lesser charge, and (e) developing theareas of lesser charge with electrostatically charged toner particleshaving the same polarity as that of the charged layer.
 2. A processaccording to claim 1 wherein the layer on the conductive support is aphotohardenable layer.
 3. A process according to claim 2 wherein thelayer is photopolymerizable.
 4. A process according to claim 1 whereinthe layer on the conductive support is a wash-out photohardenable layer.5. A process according to claim 1 wherein the layer on the conductivesupport is a leuco dye-containing photosensitive layer.
 6. A processaccording to claim 1 wherein the layer on the conductive support is asilver halide-based photosensitive layer.
 7. A process according toclaim 6 wherein the silver halide-based layer consists essentially of asilver halide photographic salt dispersed in a synthetic insulatingpolymeric binder that is swellable in aqueous solutions having a pHgreater than about 81/2.
 8. A process according to claim 1 wherein thelayer on the conductive support is a silver halide-based layer preparedfrom a diffusion transfer film comprising development nuclei dispersedin a synthetic insulating polymeric binder that is swellable in aqueoussolutions having a pH greater than about 81/2.
 9. A process according toclaim 1 wherein the exposed layer is charged by corona discharge.
 10. Aprocess according to claim 1 wherein the charged layer having imagewiseareas having different rates of charge decay is allowed to stand for0.001 to 10 minutes to form an electrostatic image corresponding to theimagewise generated areas.
 11. A process according to claim 1 whereinthe charged layer having imagewise areas having different rates ofcharge decay is allowed to stand for 0.01 to 0.25 minute to form anelectrostatic image corresponding to the imagewise generated areas. 12.A process according to claim 1 wherein the imagewise generated areashaving different rates of charge acceptance are charged and anelectrical field is immediately created to attract toner particlespreferentially to the areas of lesser charge.
 13. A process according toclaim 1 wherein the electrical field to attract toner particlespreferentially to the areas of lesser charge is created by providing avoltage on the development electrode or the conductive backing of theelectrostatic element that is less than the potential in the chargeretaining areas of the electrostatic element.
 14. A process according toclaim 1 wherein the developing is accomplished with a dry electrostatictoner.
 15. A process according to claim 1 wherein the developing isaccomplished with a liquid electrostatic developer.
 16. A processaccording to claim 15 wherein the liquid electrostatic developerconsists essentially of (a) a nonpolar liquid having a Kauri-butanolvalue of less than 30, present in a major amount, (b) thermoplasticresin particles having an average by area particle size of less than 10μm, and (c) a nonpolar liquid soluble charge director compound.
 17. Aprocess according to claim 1 wherein the developed image is transferredto a receptor support.
 18. A process according to claim 16 wherein thedeveloped image is transferred to a receptor support.
 19. A processaccording to claim 17 wherein the receptor support is paper.
 20. Aprocess according to claim 18 wherein the receptor support is paper. 21.A process according to claim 17 wherein the transfer is accomplished byelectrostatic means.
 22. A process according to claim 18 wherein thetransfer is accomplished by electrostatic means.
 23. A process accordingto claim 3 wherein the photopolymerizable layer comprises of an organicpolymeric binder, at least one compound having at least oneethylenically unsaturated group, and a photoinitiator.
 24. A processaccording to claim 23 wherein the photopolymerizable layer contains achain transfer agent.
 25. A process according to claim 23 wherein thephotopolymerizable layer contains an organic compound selected from thegroup consisting of at least one organic electron donor, at least oneorganic electron acceptor, and a substituted aromatic amino compoundwith or without a strong acid.
 26. A process according to claim 23wherein the exposed photopolymerizable layer is charged by coronadischarge.
 27. A process according to claim 23 wherein the developing isaccomplished with a dry electrostatic developer.
 28. A process accordingto claim 23 wherein the developing is accomplished with a liquidelectrostatic developer.
 29. A process according to claim 28 wherein theliquid electrostatic developer consists essentially of (a) a nonpolarliquid having a Kauri-butanol value of less than 30, present in a majoramount, (b) thermoplastic resin particles having an average by areaparticle size of less than 10 μm, and (c) a nonpolar liquid solublecharge director compound.